This invention relates to improvements in an exhaust gas purifying catalyst and a method of producing the catalyst, and more particularly to the exhaust gas purifying catalyst for removing hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine of an automotive vehicle, a boiler or the like, particularly for effectively reducing NOx in exhaust gas in an oxygen-excessive region or lean region, and the method of producing such a catalyst.
Hitherto, automotive vehicles of a low fuel consumption have been eagerly desired from the view points of exhaustion of petroleum resource and warming-up phenomena of the earth. Regarding automotive vehicles with a gasoline-fueled engine, attention has been paid on development of automotive vehicles provided with a so-called lean-burn engine. In the automotive vehicles provided with the lean-burn engine, exhaust gas is in an oxygen-excessive or lean region (atmosphere) in which an air-fuel (air/fuel) ratio is leaner or larger than a stoichiometric value. In case that a usual three-way catalyst is used in the lean region, removing or reducing NOx can be insufficient under the effect of the excessive oxygen. Accordingly, development of a catalyst for effectively reducing NOx even under an oxygen-excessive condition has been desired.
A variety of catalysts for reducing NOx in such a lean region have been proposed. One of them is disclosed in Japanese Patent Provisional Publication No. 5-168860 in which Pt and lanthanum (NOx adsorbing or trapping agent) are carried on a porous carrier so that NOx is adsorbed in the lean region and released in a stochiometric region in which exhaust gas has a stoichiometric air-fuel (air/fuel) ratio.
However, fuel and lubricating oil used in the engine contains sulfur (S) which is to be discharged in the form of oxide from the engine. Accordingly, the NOx adsorbing agent is subjected to poisoning with sulfur, and therefore lowering in NOx adsorbing ability occurs in the NOx adsorbing agent. This is a so-called sulfur-poisoning.
In order to protect the NOx adsorbing agent or material from such sulfur-poisoning, the present inventors have proposed the following technology in Japanese Patent Provisional Publication No. 2000-54825: A catalyst has a catalytic layer which includes an inner layer, and a surface layer formed on the inner layer. The surface layer contains a S adsorbing material (such as magnesium Mg) which can adsorb sulfur but readily decompose sulfur. The inner layer contains a NOx adsorbing material (such as barium Ba).
Additionally, Japanese Patent Provisional Publication No. 7-132226 discloses a technology in which the concentration of alkaline metal or alkaline earth metal is lowered in a direction from the upstream side to the downstream side relative to flow of exhaust gas.